1. Technical Field
This invention relates to insulating units composed of an isocyanate based foam injected into a cavity of cabinet comprising an inner and outer liner and a process for production of such insulating units.
2. Background of the Art
One of the most commercially important applications for rigid polyurethane foams is in the appliance industry. In this application the foams supply insulation from heat and/or cold, and may also serve to increase structural integrity and/or strength of the appliance. Frequently the foams are part of composite, sandwich-type constructions wherein at least one outer layer of a rigid or elastic material, such as, for example, paper, plastic film, rigid plastics, metal sheeting, glass nonwoven materials, chipboard, and the like is also included. In particular applications, such as refrigerators and freezers, the components of the rigid polyurethane foam may be injected into cavities, wherein the components first fill the cavity and then complete reaction to form the final rigid polyurethane foam. In order to ensure the necessary characteristics of the final foam in cavity-filling applications, it is particularly desirable to complete introduction of the foam-forming components within a relatively short time. Another important requirement for the rigid polyurethane foam is a good adhesion to the refrigerator cabinet or freezer housings.
An industrial process to manufacture refrigerators and freezers is described in PU Handbook by G. Oertel, Hanser publisher (1985). The refrigerator/freezer cabinet has a cavity of relatively complicated design with the housings made of metal and/or plastics. Fixtures support the prepared shell (outer wall surface) and liners (inner wall surface) of the housing against the resulting foam pressure and maintain the proper distance between them. The process generally involves injection of all the liquid reactants into the unit at one time as pouring of further liquid reactants into already rising foams can lead to imperfections and poor cell structure. In such processes, normal changes in atmospheric air pressure may strongly influence the foaming process.
In general, heat- and cold-insulating rigid polyurethane foams used as insulation in such units may be produced by reacting organic polyisocyanates with one or more relatively high viscosity compounds containing at least two reactive hydrogen atoms, such as polyester- and/or polyether-polyols, usually in the presence of low molecular weight chain extenders and/or cross-linking agents, in the presence of non ozone depleting blowing agents and catalysts. If desired, auxiliaries and/or additives may be further included. Choice of appropriate components enables production of rigid polyurethane foams having acceptably low thermal conductivity and desirable mechanical properties.
For example, Canadian Patent 2,161,065 discloses use of a formulation including components that contain, alone or in combination, at least 32 percent by weight of aromatic radicals. It is asserted therein that the relatively high aromaticity of the formulation serves to reduce the insulating performance (thermal conductivity) by at least 0.5 mW/mK, and also improves flame resistance and aging behavior of the foams.
The proper selection of blowing agents in such application has often been problematic. While chlorofluorocarbons (CFCs) have long been known to perform well in insulating foams, their use is progressively more restricted by law for environmental reasons. Thus, a body of art has arisen with the goal of reducing or eliminating CFC use while still achieving, or attempting to achieve, desirable insulation and mechanical performance close to the performance obtained with CFC's. This is particularly important because, as a general rule, the blowing agent remains in the rigid polyurethane foam for a considerable time as a cell gas. Thus, the cell gas itself, and not just the foam matrix, provides a very significant portion of the overall insulation performance of the foam. This is particularly so in applications such as appliances, where the generally very slow diffusion rate of the gas out of the cells is further slowed, or virtually prevented, by encasement of the foam in plastic or metal outer layer(s).
While selection of components of a foam-forming formulation is important in determining the insulating capability of a final rigid polyurethane foam, those skilled in the art have also had to address process-related issues, particularly as they relate to how processing variations affect the insulating and mechanical capabilities of the foams. Achieving optimum foam density, cell size, and especially uniformity, while also ensuring excellent cavity-filling or mold-filling performance, has challenged the industry to search for new ways to introduce the formulation components. For example, introduction may be achieved by single shot injection, simultaneous injection at multiple sites, positional variations of the mold or of a “cabinet,” i.e., the container having the cavity that is destined to be filled by the polyurethane foam, and the like. The speed of movement of the formulation throughout the cavity relative to the rate of reaction may also be an important factor. The faster the foam gelation, the shorter is the gel (or string) time, hence it is more challenging to fill the cavity without voids due to the fast viscosity build-up of reactants. Short gelation times are also linked to fast tack free time, which presents challenges in obtaining proper flow of foam forming formulations and foam adhesion to walls of an insulating housing unit which contain one or more restrictions or angles.
Another issue of increasing importance is a reduction in the amount of amine catalyst used in the production of fast reacting polyurethane foam as such catalyst are potentially volatile which may contribute to environmental and safety issues.
To aid in distribution of a foam forming formulation in insulating units, foaming under reduced pressure has been proposed. For example, WO 2007/058793, describes a method of molding rigid polyurethane foams wherein a density/lambda (density/thermal insulation) ratio of 1.65 to 2.15 is achieved under a pressure of 300 to 950 mbar and a packing factor of 1.03 to 1.9. Still another example may be found in U.S. Pat. No. 5,439,945 A, which discloses foams that are prepared under a reduced pressure and subsequently encased in a material which prevents ambient air from entering the cell voids. The gas within the foam reaches equilibrium at a lesser pressure than in prior systems.
Despite the multitude of approaches to these problems, there remains a need in the art for insulating housing units and a process for the production of such units that enables safe, efficient, cost-effective production of closed cell rigid polyurethane foams as an insulating barrier that attain desirable molded densities and insulation factors while filling a cavity without voids and at the same time offering good mechanical properties, fast demoldability and good adhesion to refrigerator housings.